Network segment dropping connectivity

I seem to be having a bit of a quandary and I was wondering if any network cable / electrical gurus might be able to shed some light on the problem that I am having.

It seems that as of late there is one particular segment of our network which just stops working out of the blue. This has happened twice, now. Here are some of the details:

- This segment uses standard CAT5e UTP cable.
- This cable serves as an uplink between 2 HP switches - a 16-port JE005A (Foundry office) and an 8-port J9449A (QA Office). I would guess that this segment is around 125 to 175 feet long.
- The run is in a fairly noisy environment (a foundry) - it currently crosses some electrical wiring and is in proximity by a few metres to a Thermfire sand reclamation machine, however activity of this machine does not seem to correlate with the failure of the network segment.
- All other connections on both switches still function normally.

When the failure first happened, the link lights went out on both switches. Moving the the cable to a different port on the switch at either end did not fix anything. The cable's conectivity tested fine, but since I had ample slack at both ends, I re-crimped both ends - the problem remained.

I replaced BOTH switches with new respectively identical backup models. The result was the same. The network uplink segment would not link up while everything else still seemed to work normally. (All devices on their respective switches could still talk to each other - the failure was strictly with the uplink segment.) I put the original switches back.

For one fleeting moment the link appeared to come back but the interface on the switches indicated that the link was at 10Mb rather than its usual 1000Mb. If I tried to ping one switch from the other, the response time was all over the map, from several hundred milliseconds to timing out. This lasted for a couple minutes and the segment link went dead again.

So I decided to experiment and swap out the 16-port in the foundry office with the new 8-port backup switch. When I did this, it didn't work at first, but when I picked up the little 8-port switch in my hands, the link came on. I set it back down, and the link went out again. The thing to note here is that when I set it down, it was directly on top of the existing 16-port swtich - metal on metal. I put a foam mat in between the two switches, and the uplink worked again. I could then uplink the this 'middle' switch to the 16-port and everything was back up and running like usual again, except I now have a third switch bridging the gap between the two originals.

A point of interest - a few days later I tried plugging the uplink back directly into the 16-port, and it worked fine again. Whatever noise or interference (I'm assuming this is what it was) had settled down the the point where it was no longer a problem for awhile. I am interested in hearing if anyone can tell from this description what the most likely cause of the problem was. I get the impression that there is some sort of grounding noise or issue of some sort at play here, but if anyone can shed some light I would appreciate the input.

I spoke to our electricians and unfortunately they do not seem to have equipment which can measure noise on the network cables here. They suggested that running an outdoor-rated cable outside of the building might help - however, is this where the problem lies? The thing that throws a wrench into the works for me is the fact that 'grounding' the one switch to the other by setting it on top of it caused the link to go down, but removing that ground restored the link to working condition. I have no idea if that says anything meaningful about the source of the problem.

Well, as per your description, there is a slight possibility it has to do something with randomness...too. But if you are pretty sure that grounding a switch renders link useless, then your problem lies most probably in mechanical failure of the uplink cable.
I doubt it being interference, or better, induction from surrounding machines, as it sounds like current is flowing through one of the cooper wires to the ground.

Uplink falling to 10Mbit is the indication that wires are cut or have bad conductivity. Might be induction, too, but I doubt. Rather it is insulation poor on some region, letting moist in, so your problems might also be weather-related.

Installing an external grade cable won't make much difference I don't think, you would need a shielded cable not an external one. Ideally you need to get the cable tested to confirm if there is interference on it. The cable may even be damaged if its in an active work area.

Would it be possible to re-route the cable or to install a new cable away from any of the power cables?

From what you have said, it does sound like either an electrical issue or a fault with the cable. Both can be tested by a qualified cable installer.

Were both of these switches 100MB or 1000MB? I've came across a similar issue before where the switches were gigabit but the link would go up and down constantly due to the length and noise on the cable. Finally hardcoding both switches to 100MB Full Duplex allowed the link to stay up without any intermittent drops.

I know this does address the electrical side of things but I just wanted to be sure I understood the problem fully. Also as you mentioned the cable that is running between these two switches is going over some electrical wiring which is not recommended and could be causing the problems you are having. The switches may just be handling the problem a bit differently.

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LostInThoughtAuthor Commented: 2013-05-28

Thank you for the responses. The general consensus seems to be that the existing cable is the problem and that running another cable away from the electrical interference would be advised. This is probably the cheapest option for us at the moment - we don't really have anyone available who can do a proper test on the existing cable for noise/interference.

As far as shielded cabling goes, my initial bit of research seems to indicate that we would require not just that one segment to be shielded in order for it to be effective, but all the equipment would also need to be properly grounded as well, and given our current layout, that could be a bit problematic - this is a very old building which I am sure also needs electrical improvements, but this is the first real problem we've run across.

It is possible to run another cable, but the only realistic way for us to keep it away from electrical wiring would be to run it outside the building.

I had not thought of manually setting the speed to 100Mb on both switches - I will keep that in mind as a possible option. For now I will put in a service request to have the electricians run another cable outside the building. It is just that from what I have read, I have my doubts whether using STP cabling just for that segment alone will help.

Perhaps running a fiber-link could be an idea.
Fibers will not pick up noise.
Fibers isolate.

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LostInThoughtAuthor Commented: 2013-05-31

We will be running another cable away from the electrical wiring, so I am awarding points to those that suggested re-running wire, but also giving points to BigPapaGotti because the manual setting of the switches to 100Mb seems to have really helped in the interim - I feel silly for not having first tried it myself. Oh well, that's what the experts are for!

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